1. Field of the Invention
This invention relates in general to semiconductor devices and more particularly, to semiconductor light emitting devices such as light emitting diodes for use in color displays, and laser diodes for optical recording and printing systems.
2. Description of the Related Art
Compound semiconductors in general and AlGaInP semiconductor materials in particular have recently received considerable attention as a group of new semiconductor materials for use in high brightness light sources such as, for example, light emitting diodes for color displays, having emission wavelengths ranging from green to red, and semiconductor laser diodes in the visible wavelength region for optical recording and printing systems.
The AlGaInP semiconductor materials have a largest band gap energy among the direct transition III-V alloy semiconductors which is lattice-matched to GaAs. The maximum band gap energy reaches about 2.3 eV, which corresponds to 540 nm in wavelength.
When a heterojunction with AlGaInP material is formed, however, a relatively small conduction band discontinuity (.DELTA.Ec) results between an active layer (or light emitting layer) and a cladding layer which has a band gap energy larger than that of the active layer. This small band offset causes injected carriers (electrons, in this case) to overflow with relative ease from the active layer to the cladding layer, resulting in disadvantages such as, for example, a large variation of laser threshold current density with temperature, or unsatisfactory temperature characteristics of the light emitting devices fabricated with the materials.
To achieve a satisfactory carrier confinement, and thereby overcome the above-mentioned difficulty, a structure has been disclosed in Japanese Laid-Open Patent Application No. 4-114486/1992, in which a quantum barrier structure is formed between an active layer and a cladding layer.
To fabricate a semiconductor laser, it is necessary to have a structure such that the confinement of carriers and emitted light into an active layer is carried out by a cladding layer, which has a band gap energy large enough for the confinement.
However, because of the small band offset of the conduction band mentioned above, the magnitude of a band gap energy of the active layer material is somewhat limited, and a material with too large a band gap energy can not be used for the active layer in conventional doublehetero (DH) structures composed of conventional bulk materials.
In addition, although the addition of Al in AlGaInP semiconductor materials is generally known to result in large band gap energies, a smaller amount of Al addition is preferred because of the highly reactive nature of Al, which reacts even with an even minute amount of oxygen in source materials and/or ambient atmosphere during layer growths. This may form deep level impurities and decrease the light emission efficiency.
Several semiconductor laser diodes have been reported, consisting of GaAs substrates and AlGaInP active layers which are lattice-matched to GaAs substrates.
For example, a laser diode comprises an (Al.sub.0.19 Ga.sub.0.81).sub.0.51 In.sub.0.49 P active layer and has an emission wavelength of as short as 632.7 nm continuous at room temperature. To further decrease the emission wavelengths and threshold current densities, a quantum well structure is disclosed in Japanese Laid-Open Patent Application No. 6-77592/1994, consisting strained quantum well layers used as an active layer.
As another example, a laser diode is described by Hamada et al. in Electronics Letter, Vol. 28, No. 19 (1992), pages 1834-36. As disclosed therein, the laser diode consists of compressively strained (Al.sub.0.08 Ga.sub.0.92).sub.0.45 In.sub.0.55 P multiquantum wells (MQW) as active layers incorporating multiquantum barriers (MQB), and has a continuous laser emission at 615 nm at room temperature. However, temperature characteristics of the laser diode is not satisfactory for practical use.
As above-mentioned, laser diodes consisting of conventional materials which are lattice-matched to GaAs substrate, have shortcomings such as difficulties in decreasing emission wavelengths and unsatisfactory temperature characteristics, thus being incapable of having laser emission wavelengths at about 600 nm and less.
As still another example, a laser diode is described in Japanese Laid-Open Patent Application No. 6-53602/1994, which consists of a GaP substrate, an Al.sub.y Ga.sub.1-y P (0.ltoreq.y.ltoreq.1) cladding layer, and a Ga.sub.x In.sub.1-x P (0&lt;x&lt;1) direct transition MQW active layer doped with nitrogen as isoelectronic trap impurities, incorporating Ga.sub.x In.sub.1-x P (0&lt;x&lt;1) barrier layer. This laser diode has an emission wavelength as small as about 600 nm.
Also disclosed in Japanese Laid-Open Patent Application No. 5-41560/1993 is a laser diode which consists of a GaAs substrate, a double-hetero (DH) structure of (AlGa).sub.a In.sub.1-a P (0.51&lt;a.ltoreq.0.73) layers formed on the substrate, and further provided with a GaAs.sub.x P.sub.1-x buffer layer disposed on the substrate and under the DH structure, to relax a lattice-mismatch caused in the region between the GaAs substrate and the DH structure.
Although these structures have advantages such as, being fabricated with materials of fewer aluminum contents and still capable of attaining short laser wavelengths, they also have shortcomings, such as difficulties in confining enough carries in active layers.
FIGS. 1a and 1b represent energy band alignments for devices fabricated on GaP substrates which are described in Japanese Laid-Open Patent Application No. 6-53602/1994. For constructing the band alignments, a reference was made to the description by Tiwari et al. in Applied Physics Letter Vol. 60 (1992), pages 630-32.
Referring to FIG. 1a a device consists of a GaP substrate, a GaP cladding layer, and a Ga.sub.0.7 In.sub.0.3 P active layer. From the above-mentioned band alignment, a conduction band offset (.DELTA.Ec) is expected to be about 100 meV and a valence band offset (.DELTA.Ev) is about 0 meV, in this construction.
In FIG. 1b there are shown a cladding layer composed of AlP, and an active layer of Ga.sub.0.7 In.sub.0.3 P disposed on a GaP substrate. In this construction, a valence band offset .DELTA.Ev is about 470 meV, and a conduction band energy of Ga.sub.0.7 In.sub.0.3 P is higher than that of the AlP cladding layer by about 190 meV, in contrast to the structure of FIG. 1a.
When a cladding layer is disposed using a GaP-AlP alloy, a heterojunction may be formed, which may have an energy band offset of a magnitude large enough to confine both electron and hole carriers, based on the above-mentioned consideration of the band alignment. In practice, however, .DELTA.Ec for this system is less than 100 meV, which is not large enough to achieve an electron confinement sufficient for the practical device application.
Although it is known that .DELTA.Ec increases by adding Al in place of Ga and In, the magnitude of the increase in .DELTA.Ec is minimal. In addition, a lattice-mismatch between the AlGaInP active layer and the GaP substrate increases further from the present value of 2.3%. This increase of the lattice-mismatch effectively decreases the critical thickness, thereby requiring a smaller layer thickness during fabrication.
Since the critical thickness is defined as the minimum thickness to obviate occurrence of misfit dislocations caused by the lattice mismatch, the above-mentioned increase is not preferable to the practical application.
The present argument on the band offsets and critical thickness is also true for the aforementioned (AlGa).sub.a In.sub.1-a P device described in Japanese Laid-Open Patent Application No. 5-41560/1993.
The above stated structures therefore have advantages such as, being fabricated without aluminum and still capable of attaining shorter laser emission wavelengths. However, no heterojunction has been found to have both .DELTA.Ec and .DELTA.Ev band offsets large enough for the practical application, such as .DELTA.Ec of about 190 meV or higher, and .DELTA.Ev of about 60 meV or higher.
Furthermore, to fabricate a laser diode on a silicon or GaP substrate, nitrogen-containing III-V alloy semiconductors such as InNSb and AlNSb, are disclosed in Japanese Laid-Open Patent Application No. 7-7223/1995. In that disclosure, the band gap energies of these two semiconductors, InNSb and AlNSb, are estimated by linearly interpolating band gap energies of InN and InSb, and AlN and AlSb, respectively, to find that AlN.sub.z Sb.sub.1-x with x=0.4 is lattice-matched to GaAs, and that has a band gap energy of about 4.0 eV.
If the alloy semiconductor mentioned just above is feasible, light emitting devices may be fabricated, which have emission wavelengths ranging to the ultraviolet spectral region. However, since almost all of these nitrogen-containing alloy semiconductors are in the non-miscible region in the solid solubility diagram, they are not feasible by conventional crystal growth methods but only by non-equilibrium growth methods such as, for example, metal organic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE).
Even by MOCVD and MBE, the nitrogen content has not been able to exceed 10%, and a content of about 40% preferable to the device application is quite difficult to achieve. In addition, as described in the aforementioned Japanese Laid-Open Patent Application No. 6-334168/1994, a relatively large degree of energy level bowing is present due to a large electronegativity of nitrogen. Therefore, their band gap energies decrease by adding more nitrogen into InSb or AlSb, and at the alloy composition for which the lattice-matching to GaAs or Si is achieved, its band gap energy is smaller than those of InSb or AlSb, which is in contrast to the above-mentioned expectation.
Accordingly, it is difficult to form an alloy semiconductor such as described in Japanese Laid-Open Patent Application No. 6-37355/1994. Namely, by the use of the energy band bowing, a light emitting device with 1.5 micron emissions may be achieved with a GaInNAs material formed on a GaAs substrate. However, light emitting devices of shorter wavelengths can not be achieved by these structures.